1. Field of the Invention
This invention relates to an attraction holding device for accurately holding a measurement object in an interferometer or the like.
2. Description of the Prior Art
Interferometers such as flatness testers for measuring the flatness of surfaces of measurement objects are widely used, for example, for inspecting the surface flatness of silicon wafers used as a material of integrated circuits (ICs) prior to the IC manufacturing process. In order to accurately measure the surface shape of a measurement object, it is necessary not only to adjust the interference optical system but also to securely mount the object on the interferometer so that the object may not be distorted. Also when the surfaces of parts or objects are processed or machined precisely, for example, in the manufacture of ICs, it is required to securely hold the objects without deforming the objects.
In order to satisfy the aforesaid requirements, there has heretofore been used a method in which the object is held by attraction on a reference plane exhibiting high flatness. Specifically, attraction holes are perforated through the reference plane, the object is placed on the reference plane, and then the object is attracted to the reference plane by a vacuum through the attraction holes.
In the method as described above, an important factor is how to maintain the accuracy of the reference plane in the fabrication of the holding device. Usually, the reference plane is not obtained by cutting and grinding as a structure integrated with a surface plate used as a measurement stage or a holding state, and the like, but instead the reference plane is fabricated separately from the stage for reasons of the formation of the attraction holes and the manufacturing process. In this case, a reference plate on which the reference plane is formed must first be manufactured at a high accuracy, and then accurately mounted on the stage. Heretofore, however, though the reference plate could be manufactured at a high accuracy, it was not always possible to simply judge whether the reference plate was mounted on the stage at a high accuracy or not immediately after the reference plate mounting step. That is, even when fine dust intervened between the stage and the reference plate, it was impossible to find the existence of the fine dust during or immediately after the mounting step, but instead the existence thereof could be found only by the final inspection. Thus the aforesaid method is not an efficient one.